Troubleshooting routers is common in networks. Both dedicated routers such as
Cisco routers and software such as Windows can perform the routing functions.
Routers are used to connect multiple networks together to form an internet. Note
the appearance of the router icon.

Windows servers and workstations know the
address of the router to be the Default gateway address configured in the
network properties. Routers work with the information in the network layer
protocol to do routing. The router determines whether each message is

a local message or

one destined for some other network in the
internet.

Local messages are not forwarded by a router.

10.1.1.40

Router does not forward local message

To 10.1.1.41

Messages destined for other networks are
forwarded by a router.

10.1.1.40

Router does forward remote message

To 11.1.1.41

To 11.1.1.41

The network layer includes network addresses
that the router can use to forward a message in the direction of the destination
network. In the case of IP, the netid network identifier is the front part of
the IP address. The following diagram shows how routers only need to
understand up to the network layer to do their routing function.

Client

Server

HTTP

HTTP

TCP

Router

Router

TCP

IP

IP

IP

IP

IP

IP

Ethernet

Ethernet

PPP

PPP

Ethernet

Ethernet

The following diagram shows the IP and MAC
addresses for a packet as it is passed from one network to another through two
routers. You can see that the IP packet stays consistent but the Ethernet frame
is replaced by the routers as they pass the IP packet from one network to
another.

A

B

IP address: A to B
MAC address: A to R1

R1

IP address: A to B
MAC address: R1 to R2

R2

IP address: A to B
MAC address: R2 to B

RIP Routing Information Protocol

Routers learn about each other and other
networks through a routing protocol. RIP is the simplest routing protocol and is
usually the default with routers. Routers use routing tables to determine where
to send forwarded messages. Routers know the network addresses of all networks
that they are physically connected to because each network interface required a
network address configuration. Routers do not have any information about remote
networks configured by default. Although manual configuration is possible, most
routers use a routing protocol to automatically discover the location of remote
networks. With a routing protocol, routers can automatically reconfigure
themselves in the internet topology changes.

RIP is rather simplistic. RIP routers broadcast
their known routing tables on all network connections. The routing table is
initialized with the network addresses of all interfaces. Initially, remote
networks are not known, but as RIP messages from remote routers are received,
the router add to its knowledge of remote networks.

Initially, routers only have knowledge of their
directly connected network addresses in their routing tables.

Routing table
10.1.0.0
10.2.0.0

Routing table
10.2.0.0
10.3.0.0

10.1.0.0è

ç10.3.0.0

ç10.2.0.0

10.2.0.0è

Network 10.1.0.0

Network 10.2.0.0

Network 10.3.0.0

Once they have learned about the remote network
addresses from RIP messages, they can update their router tables. The update
router information will be broadcast so that other routers will learn about
these updates also.